Dimethyl carbonate (DMC) is a novel “green” chemical product to which great attention is paid both domestically and internationally. Since DMC comprises methoxy group, carbonyl group and carbonyl methyl oxygen group in a molecule, it exhibits high reactivity, and could be used to replace for highly toxic phosgene as carbonylating agent and for dimethyl sulfate as methylating agent. DMC can be used as raw material to directly synthesize food additives, anti-oxidants, plant protection agents, high performance resins, fuels, pharmaceutical intermediates, surfactants, etc., thus it is praised as a potential “novel base block” in organic synthesis. Furthermore, since DMC possesses relatively high oxygen content as well as suitable vapor pressure, water resistance and mixing distribution coefficient, it could serve as an ideal gasoline additive. This further extension of the application of the DMC will surely make DMC a new economic growing point in the chemical industry, and is of practical significance due to huge potential demands to DMC.
DMC was mainly synthesized by phosgene method in conventional method. High toxicity of raw material phosgene and corrosiveness of chlorine ion limited the large-scale production and application of DMC. In 1983, Enichem Company in Italy developed a non-phosgene method to synthesize DMC by oxidative carbonylation of methanol in liquid phase [Romano U., Tesei R., Mauri M. M. et al, Synthesis of dimethyl carbonate from methanol, carbon monoxide, and oxygen catalyzed by copper compounds, Ind. Eng. Chem. Prod. Res. Rev., 1980, 19: 396-403; Micheal A. P., Christopher L. M., Review of Dimethyl Carbonate (DMC): Manufacture and its Characteristics as a Fuel Additive. Energy and Fuels 1997, 11, 2-29], which brought the synthesis of DMC to a new stage. In 1992, UBE Industries Ltd. in Japan developed a method of gas-phase oxidative carbonylation of methanol [J. Kizlink, Collect. Czech. Chem. Comm. 1993, 58, 1399; Y. Sasaki, Chem. Lett., 1996, 825; S. T. King, Reaction mechanism of oxidative carbonylation of methanol to dimethyl carbonate in Cu—Y zeolite, J. Catal. 1996, 161, 530-538], and this made the synthesis of DMC quickly commercialized. However, since the catalyst used in said method comprises CuCl as main active component, the catalyst has strong corrosion to facilities and short life, and the method suffers expensive raw material gas and toxicity of CO. Another non-phosgene method for preparing DMC is called transesterification method [Knifton J. F., Duranleau R. G., Ethylene glycol-dimethyl carbonate cogeneration, J. Mol. Catal., 1991, 67: 389-399; Nishihara K., U.S. Pat. No. 5,292,917, 1993; Tatsumi T., Watanabe Y. and Koyano K. A., Synthesis of dimethyl carbonate from ethylene carbonate and methanol using TS-1 as solid base catalyst., Chem. Commun. 1996(19): 2281-2282], wherein CO2 reacts with ethylene oxide or propylene oxide in the presence of a catalyst to form ethylene carbonate or propylene carbonate, and then ethylene carbonate or propylene carbonate is subjected to transesterifying with methanol to form DMC and ethylene glycol or propylene glycol. In comparison with other synthetic methods, this process has the advantages of cheaper raw materials, lower toxicity of raw materials, no three wastes, high yield, and low corrosion, and the by-product, ethylene glycol or propylene glycol, can be recovered. However, this method has at present the disadvantages that the catalyst used has lower activity and shorter life, reaction conditions are severe, and organic solvents are used during the reaction so that the subsequent separation of the products is difficulty and thus facility investment as well as energy consumption is increased. Therefore researching and developing new reaction approach to further raise technological economical efficiency and technological operability will be of importance.
In order to overcome the above-mentioned disadvantages, the present inventors have developed a novel process and a novel technique for the direct synthesis of DMC from urea and methanol, as disclosed in Chinese Patent Application No. 01130478.2 entitled “Method for preparation of dimethyl carbonate from urea and methanol” and in Chinese Patent Application No. 01131680.2 entitled “Method for preparation of dimethyl carbonate from urea and methanol using heterogeneous catalyst”. In such a method, since the raw materials, urea and methanol, are common chemical raw materials, their prices are rather low, and thus raw material cost is lower. In addition, the method has other advantages such as safe and simple process, higher activity of reaction and higher selectivity of the product, and thus could significantly lower the production cost of DMC. However DMC yield of said process is still a little lower and needs further improvement.